Automatic brightness and contrast control circuit



A. U. SHARON Jan. 5, 1965 AUTOMATIC BRIGHTNESS AND-CONTRAST CONTROL CIRCUIT Filed D66. 5, 1 961 INVENTOR:

L u.sH non,

ls ATTORNEY.

tube.

e provernent in a circuit, including a lightresponsive element, which provides a; control voltage for controlling 15 .these changes.

V an improved circuit for -p'rovidingautoma 'c co i ing ofthe electronbeam occurs.

AUTOMATIC BRIGHTNESS AND CONTRAUST4 CONTROL CIRCUIT i i Al U. Sharon, Liverpool, N.Y.,assignor to General Elec- 5 tric Company, a corporation of New York Filed Dec. 5, 1961, Ser. No. 157,114 f- ,--4 Claims. (Cl.'178 7.5)

This invention relates to a means for automatically alteringthe `brightness and contrast ofra reproduced image "10 in -a television receiver responsive to variations inV the intensity of thel light level striking the face of a picture More particularly, the invention relates to `an im- A circuitfor automaticallycontrolling image contrast and brightness in a televisionreceiver isdescribed in cop pending application, Serial No." 104,909, tiledApril 24,

1961, which is assigned to thetassignee'ofthis invention; 2Q

in block form, of a television receiver embodying the im- It is therein disclosed that anautomatic control'voltage for altering contrast and brightness is derived' from a cir` cuit which includes an'element having an electrical characteristic which varies with the intensity oflight striking the element. Contrast ,is automatically varied'by coul 25 pling the control voltage to an automatic gain control cira. cuit in the television receivertin order to vary 'a preselected and regulated video signal level'at an inputelectr'ode of' the picture tube. 'Ihe controlV voltage is also coupledto an electrode of the picturetube for 'simultaneously vary- 30 ing image brightness.

- In the aforementioned circuit, thepreselected andr'egu-v Y lated video signal level atan input electrode of the picture tube is varied by changing the'cathodevoltage andtherefore the bias on an amplifying device in the automatic 35v sequently subjected to relatively high power dissipation 40 which tendsto varytheV` characteristics of theel'ement and to' shorten itsusefullife. Althoughlightvresponsive elep ments with largerpower ratings may be -utilizedtovercorne this diiicultyjjsuch an alternativeis uridesirable,' especially`whe're3cost of the receiver is offpriniary import-:

brightness control in`a television receiv Itisanother object ofthis invention'top ovid an uto-'f 1 4matic brightnessrand' contrast control circuit inr variations in the characteristic of a light respons ive*ele-gV Arnentdue` to' power, dissipation ndage are substantially reduin H 55 In"a 'n embodiment-of 'the aforementioned brightness` :and contrast control-circuit disclosed in" the` copending application, brightness is variedbylcoupling thecontrol voltage to the first anode of the picture tube;l i Such aco'nj 'f' nectionprovides satisfactory variations in "image brightL'Q ness only with apicture tube'havinga high.transldiridiic-` j tance. Furthermore, since the' firs'tf'anodeis af'coniponent T" of the" electron lens, some slight but4 und'esr Another object of his inventionis toprov matic contrast `and brightness control circuit elimi inates defocusing of the picture image and'ailoyvs opera` tion with conventional picture` tubes having', present day designlvalues of'transconductance.

In accordance with this invention, atelevision i'eceiver` 70 having a picture tubewith aviewing faceand ar1 aut.o

matic gain control' circuit for regulating-a-preselected' Utr-9de- 11;-: represetlt -inp 'ier-theYidoinputsgnal-i t. lates anlelectronb am vvithinthe picture tube f or "repro- 1 Aducingan imaggfon Yayviewingface 12 of the picture tube. "r r 5 a The mamar/idro. signa 4 isalso., directement ementsof higher powerar more expensivel 3,164,673 Patented Jan. 5, 1965 ffice that whichstrikes `the face of the picture tube. The ele-` ment is connected to a control circuit for providing a convtrol voltage which varies in amplitude with the light intensity. The control voltage provided is of a polarity suitable for connection both to a cathode of the picture tube for controlling brightness and to control electrode of l an amplifyingdevice in the automatic gain control circuit for controlling contrast. Impedancemeans are connected to the element for reducing variations in control voltage dueto undesirable changesjin the lumen-resistance characteristic o f the elementsV 1 Further object, features and attending advantages of .this invention willrbe apparent with reference to theY fol- `lowing specification and drawings in which:

.FIGURE 1 is a diagram, partly in schematic and partly proved automatic contrast and brightness control circuit of this invention, f t

FIGURE 2 is a diagram illustratingthe light intensity resistance characteristic of the light responsive 'element utilizedin the invention, and i i FIGURE 3 `is a frontyview of a television receiver cabi-` net illustrating the relative positions of the-viewing face of the picturettubeand the lightresponsive element.

i Referring Anow to FIGURE l, a television receiver is illustrated` wherein atransmitted television signalv received by an antennal and coupled successively to conventional,

RF tuner,-l;ampliier, converter, and amplifier stages representedby blockl andtoconventional video detector and amplitier'rstages representedgby block 3. An ampliiiedfcompos'ite video-'waveform 4 is derived from the anode circuit not shown,` ofa video amplifier included.

in .fblock S-,andis coupled to the cathode electrode 5 of a-.cathode Vray,pictu`re tube'f' via a, sliding ar'm\' 7 of la manualcontrast control potentiometer `8, a coupling circuit 9 and, a ..peal rin g Circuiti 1 0.V Although waveform 4 is vshown coupledgto the cathode electrode 5, it--coild-i when properA polarities arecbserved, be-coupled traanother inputtelectrode Yof the-picture tube as arco'ntrol.` electrode '11, Ei'ther the cathodeS or the controlfelec-Q .led ,r0dfs--,af-fthel picturertuber-A- coupled to an inptcontrol electrode 20ct' a keyed auto-` ,matic*gain controlfamplifying" triodewZl` via a voltage divider network; comprisingafpair of resistors 2 2 and 23..

A catiiode 24 offthelge'yer ode `21 is biased Vat a positive potential by a voltage divider comprising a resistor 25- and a potentiometer 2,6 which are interconnected between. a-source-of positive potential, B'1}, and ground. 1 The cathodeA -24 is-fbypassedmto ground by a capacitor -27 Since `the controlelectrode 20 isDQ, coupledto the l anode' circuit of the' Avideo ampliena 'D ,C potential ex ists at controlelectrodel and aD. C. controlelectrode toacathode bias-voltage is A therefore provided`-for lthe keyerftube 21.; This-bias voltage can bemanually'varied byadjusting ,the erm-0f pQtentiOmeter 2.6. f

Anode energy in' theytformuof positive' vo ltage pulses .Y 28 lis-derivedfrom-fthe horizontal sweep system 29 ofthe televisionreceiverand iscoupledto an anode 30'of the Vtriod'e21 via acapacitor 31: A `iiltercircuitrcomprising resistors 32and 33 and capacitor 34 is also connected to the anode 30. An automatic gain control bias is developed at the junction of filter elements 32, 33 and 34. This automatic gain control biasivoltageis connectedfto RF l' vidualfcell's.V f f '11n accordance and IF stages in the receiver via an AGC bus 35 for controlling the gain of these stages. n

The aforementioned AGC Vbias voltage is developed in a mannerwell lnown upon coincidence ,of a synchronizing component 13 of thecomposite video signal 4 Vand a t pulse 2S at'thecontrol'electrode 2li and anode 21, respec-V Ytively. This biased voltage regulates an initially pre-V t 3,164,673.l-r

selectedY video signal level `and an input electrode of the vpicture tube.v The regulatingl action maintainswthe 4video Isignal levelsubstantially constant when variations in the intensity ofthegreceived input signal occur. The bias on the keyer tube 21 establishes this level. Potentiometer 26 is provided formanually adjusting the level. lA circuit,

described hereinafter, is'V provided to automatically vary the biasresponsive to changes in the intensity of the light striking the face of the picture tube.

- The sweep system 29 also provides negative blanking pulses 36 for the picture tube which may be `derived from loading resistor V45 is provided` and is connected in parallel with the cell 39 in order" to minimize the effect 'of `the above mentioned changes in ycell resistance-lumen characteristic upon the controlvoltage. *The loading resistor conducts a currentA from `B-llvia potentiometer 38 to ground potential and thereby minimizes changes in the control voltage. VoltageY changes occurring acrossl the photo cell 39, although'reduce'd bytheloading of resistor 4S, are still of suiiicient amplitude Vto provide the desired yWhile Vit will beunderstood that the va1ue of circuit components `for the automatic contrast and brightness control circuit of thisinvention may varyin'order to satisfy individual requirements, the following circuit paa winding 37 `on a horizontaloutput transformer. The

pulses 36 are coupled from the winding vv 37 agto the control electrode 11 of the picture tube 6.V "The'winding 3'7is shown having one end connected to ground potential and .established a D.C. potential in the control Velectrode 11.V

' In accordance withthis invention, an impedance, aV

.lightresponsive element, anda source of potential are ar-` ranged forprovidi'ng Aa positive going automatic control voltage when the intensity oflight 'striking the face ofthe 'picture (tube decreases and anegative goingvoltage when the light intensity-increases@ The automatic control voltage is connected to the cathode of the picture tube for varying brightness and to controlrelectrode'of-the 'AGCS Yam-V v plifying dev ic'e forvarying"contrast;I A potentiometer 3 8 and a photo-conductive cell 39 are serially connected to a volt'a'gesource 131+ yfor yproviding this control voltagel The cell'39 may be a cadmium-sulphide plioto-conductive lcell havinga light intensity-resistance?characteristieas ZfOr'arly l other typ device having `asimilarcharacteristic. y n Since the resistance of cell 39 varies with light intensity illustratedl by the solid 'linezcurve 40 of FIGURE a'sillustratedin the curve loff FIGURE 2, -the voltage along rameters havelbeen'found to provide-satisfactory operai-y tion and are includedherein only by way of example', Y

AGC amplifying device 21g",` 1/2 .6AW8.

Light responsive cell 39 M Q Clariex photo cell'CL 505. n I

R8, v 25K.

`. Vt/liiile''Ilfhave illustrated'andcdescribed rand pointed fout ein theannexed claims certaintnovel features of my potentiometer 3 8 and "at the junction of thepotentiometer,

3S and cells-9. will vary inV accordance with` changes in the` klight intensity which strikesthe cell. The fcell 39`is positionedtuzbe 'struckjby lightff'of the same intensity which l strikes the face ofthe picturetube; FIGURE 3 illustratesi` vronemanner infwhich they cell"39f maybe positioned rela# n i*. tive'tothefviewing'face 12 ofthe picture tube16 on aV ,tele-fjVision-receivericabinet 13 ff`orfbeingstruckby light of .the

frjelativelyfhigh thereby rpermitting a relatively low cell Ycurrent'jand'corresponding reduction in cellpowerdissii pat'ionLQAswitch i4 is also provided and is fconnectedfin parallelwiththe cell `9ioreliminating automaticl conti'a'st and brightness cyo'ntrol'fwhen s o desire .f ReferringagaintoFIGURElZ,two' additional light y ix'i- 65 f tensity-resistancek characteristic curi/ c9346 and 47, are'4k shownand represent chan ges'y from thelinitial'desired" charj acteristic? curveV fiilfyfv Although the irripedancelofv potenti" orneter 38;;is .selectedto have a high enoughvaluefsoastogj substantially? reduce such changes due to" excessive :powerjg dissipation, 1thefmcharacteristic vcurvedi! may vary infthe` direction offcui'ves 465er 47 due-y to ambienty operatingy.

` n conditions, to age, or,V toinitial differences between" iridig with another?featureprihis'invemiaf invention, it. will be understood that various omissions, substitutions, and changes in theforms'iand'detailsof the system illustrated .maybe made by thoseskilled in the art without departing from the spirit ofthe invention and the scope ofthe claims. t `l e What I claim as new and desire to *secure by Letters Patent of thellnitedStatesis: l c Y i L 1. In a television receiverfhaving meansfor receiving a transmitted-signal, a picture tube having aviewing face and first andgsecond input electrodes including a Ycathode `electrode,f rneans'for"automatically 'varyingpicture contrast kand brightness'eomprising: anautom'atiojj gain conf trol circuit formaintaining the'- voltage, ',levcl 'oitV a video .f signaleoupledfto yan input electrode: of v`said picture tube at 'a p.reselectedV constant level whenyafriations in the in- 'tensity of thereceived signal oecurysaidautornatic gainVA v controleircuitincluding anfamplifyingdevice having ya control electrode; means foreproviding Vfacoi1t'rast and brightneS,.S, ,Contr'Ql voltage which Avaries in accordance with the intensity of "light s tr iking saidviewing facci said colitrol` voltage means 1 including a light-dependent element Ypositioned relative to said television' receiver in a manner for b'eing'struck by light o f substantially thefsameV intensity as ambient light which strikes thefaceof said picture tube; said, light-dependent element 'havingj an electrical. 'characteristic which is proportionaljftojthe"intensity of lightstrikingthe elementgrneans jdirecbcurrent coupling v saidvcontrol voltage to said 'cathode of said picture tube i Y for varyinrgthebrightness thereof;-and'rneansiconnecting said control-voltage to said controlelectrode ofsaid amplifying device js aidy automatic gain control circuit 2. .Thepapparatus of vclairn'l Where teristicoccur;Iv i,

t 3*.7`7In"antelevisionreceiverhavingmeans for'receiving a, I 'transmittedsi'gnah a picture vtube having a viewing face and jrstfandf second input electrodes' including la cathode electrode, Vmeans `for` automatically varying picture; autres? and Ibrigades;@mariene-sa ,keyed auwnwtiv Y .said- .element Vhas 'alight intensityresistance characteris -icv and impedance `means* are providedland-coupled in parallel with said elenient for: attenuating,variations1in'saidcontrol voltage when variations'in fsa'idflightt intensity-resistancecharac- A- gain control circuit for maintaining the voltage level of a video signal coupled to an input electrode of said picture tube Vat a preselected constant level When variations in the intensity of`a received signal occur, said automatic gain control circuit including an electron discharge amplifying device having a control electrode; a control circuit for providing an automatic contrast and brightness control voltage; said control circuit-including a source of potential, a resistive impedance and a photo-conductive element connected in series, said element positioned relative to said television receiver to he struck by light of the same intensity as that which strikes the face Vof said picture tube and having an electrical characteristic which is proportional to the intensity of light striA ing the element, and means connecting said source ,of potential to said series connected impedance and element for providing a control voltage which increases toward a value of more positive direct current potential and decreases toward a` o tube for varying the brightness thereof; and a resistance connecting said control circuit to said control electrode of said electron discharge amplifying device in said automatic gain control circuit for varying the contrast thereby. t

4. The apparatus of claim 3 wherein said resistive irnpedance connected in series with said photo-conductive element comprises a potentiometer having a sliding arm,

and said resistance connecting said control circuit to saidl cathode of said picture tube is connected to said sliding I afm.

References Cited by the Examiner UNrTED STATES PATENTS 3,027,421 3/62 Heijiigers 17a-7.5

- FOREIGN PATENTS 1,227,937 3/60 France. 5/60 France.

DAVED G. REDNBAUGH, Primary Examiner.

JOHN P. W ILDMAN, Examiner. 

1. IN A TELEVISION RECEIVER HAVING MEANS FOR RECEIVING A TRANSMITTED SIGNAL, A PICTURE TUBE HAVING A VIEWING FACE AND FIRST AND SECOND INPUT ELECTRODES INCLUDING A CATHODE ELECTRODE, MEANS FOR AUTOMATICALLY VARYING PICTURE CONTRAST AND BRIGHTNESS COMPRISING: AN AUTOMATIC GAIN CONTROL CIRCUIT FOR MAINTAINING THE VOLTAGE LEVEL OF A VIDEO SIGNAL COUPLED TO AN INPUT ELECTRODE OF SAID PICTURE TUBE AT A PRESELECTED CONSTANT LEVEL WHEN VARIATIONS IN THE INTENSITY OF THE RECEIVED SIGNAL OCCUR; SAID AUTOMATIC GAIN CONTROL CIRCUIT INCLUDING AN AMPLIFYING DEVICE HAVING A CONTROL ELECTRODE; MEANS FOR PROVIDING A CONTRAST AND BRIGHTNESS CONTROL VOLTAGE WHICH VARIES IN ACCORDANCE WITH THE INTENSITY OF LIGHT STRIKING SAID VIEWING FACE; AND CONTROL VOLTAGE MEANS INCLUDING A LIGHT-DEPENDENT ELEMENT POSITIONED RELATIVE TO SAID TELEVISION RECEIVER IN A MANNER FOR BEING STRUCK BY LIGHT OF SUBSTANTIALLY THE SAME INTENSITY AS AMBIENT LIGHT WHICH STRIKES THE FACE OF SAID PICTURE TUBE; SAID LIGHT-DEPENDENT ELEMENT HAVNG AN ELECTRICAL CHARACTERISTIC WHICH IS PROPORTIONAL TO THE INTENSITY OF LIGHT STRIKING THE ELEMENT; MEANS DIRECT-CURRENT COUPLING SAID CONTROL VOLTAGE TO SAID CATHODE OF SAID PICTURE TUBE FOR VARYING THE BRIGHTNESS THEREOF; AND MEANS CONNECTING SAID CONTROL VOLTAGE TO SAID CONTROL ELECTRODE OF SAID AMPLIFYING DEVICE IN SAID AUTOMATIC GAIN CONTROL CIRCUIT. 